Xerographic image formation



March 4, 1958 L.. E. WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION Filed June 4, 1954 3 Sheets-Sheet l 7......I NA\\\\\\\\\\\\\\\\\\\\\\\E 32 `l 25@ D; E

OSO C F1642 SOURCE y Zf//l/I//l/l/I//ll/l//l//l/lll//IA 27 INVENTOR.

LEWS EA WALKUP 3 SOURCE March 4, 1958 L. E. WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION 3 Sheets-.Sheet 2 Filed June 4, 1954 D.C. 0-SOURCE FIG INVENTOR.

LEWIS E. WALKUP FIG@ ATTORNEY March 4, 1958 L, E, WALKUP 2,825,814

XEROGRAPHIC IMAGE FORMATION Filed June 4, 1954 5 sheets-sheet s f ,WW/ZhFIG- INVENTOR. LEWIS E, WALKUP BY @WA ASW F l G 9 ATTORNEY United States2,825,814 XEROGRAPHIC MAGE FORMATION Application June 4, 1954, Serial No. 434,491 11 Claims. (Cl. Z50-49.5)

This invention relates in general to a formation of electrostatic chargepatterns such as xerographic electrostatic latent images and inparticular to methods and apparatus for the formation of such chargeimages.

According to the invention of Carlson described in U. S. 2,297,691 thereis provided a process and apparatus for electrophotography or xerographywherein an electrostatic charge is applied to the surface of aphotoconductive insulating layer, and this charge is selectivelydissipated by exposure to a pattern of light and shadow to be recorded.This selective charge dissipation results in an electrostatic latentimage corresponding in its charge pattern to the pattern of light andshadow to which the photoconductive insulating layer was exposed.Conventionally, in the art now known as Xerography, an electrostaticimage may be formed in this manner and may be utilized as desired, forexample, by development or deposition of finely divided material inconformity with the charge pattern, optionally, together with transferof the developed image to a print receiving surface.

Now, in accordance with the present invention, there are provided novelmethods, means and apparatus for the formation of an electrostaticcharge pattern or Xerographic electrostatic latent image wherein anelectric field is imposed through a photoconductive layer and to acontiguous insulating layer while the photoconductive layer is subjectto the action of a pattern of light and shadow of visible light or otheractivating radiation. The contiguous insulating surface is positionedextremely closely adjacent to the surface of the photoconductive layerand may be spaced therefrom by an extremely minute distance such as, forexample, the extremely small air space existent in a condition ofVirtual contact of one surface with another. While it is not intended tolimit the invention to any specific theory of operation it is presentlybelieved that electric charge under the influence of the applied fieldthrough the photoconductive layer and through the insulating layermigrates through the photoconductive layer, preferentially at thoseareas exposed to activating radiation, and migrates to the insulatinglayer across the minute air gap which may exist between this insulatinglayer and the photoconductive surface, again under the inuence of theapplied field. For the sake of clarity of disclosure and simplicity ofexplanation the invention will hereinafter be described in terms of thistheory of operation although it is to be clearly understood that thetheory is illustrative only and is not intended to be interpreted inlimitation of the scope of the invention.

The general nature of the invention having been set forth the inventionwill now be described illustratively in terms of the followingspecification and the drawings in which:

Fig. l is a diagrammatic view of apparatus according to one embodimentof the invention;

Fig. 2 is a diagrammatic view of apparatus according to anotherembodiment of the invention;

Fig. 3 is a diagrammatic view of an embodiment of the inventionemploying penetrating radiation such as X-rays;

Fig. 4 is a diagrammatic view of a further embodiment of the invention;

Fig. 5 is a diagrammatic view of a xerographic machine operating inaccordance with the present invention;

@ffice 2,825,814 Patented Mar. 4, 1958 Fig. 6 is a greatly enlargeddiagrammatic View of a section of apparatus according to a modificationof Fig. l;

Pig. 7 is a diagrammatic view of a further modification of theinvention;

Fig. 8 is a diagrammatic cross section of an induction electrodeaccording to a further embodiment of the invention;

Fig. 9 is a diagrammatic View of a cylindrical xerographicphotosensitive plate according to another embodiment of the invention.

In Fig. 1 there is illustrated a simplified embodiment of the invention.1n this figure, as in the others, the illustration is diagrammatic,largely because certain key members and certain important spacings areso extremely thin and small as to make descriptive presentationimpractical. The apparatus in Fig. 1 includes a photosensitive orxerographic member or plate designated 10, combining for example aphotoconductive insulating layer 12 overlying and in intimate contactwith a conductive backing member 11. Positioned immediately over thesurface of the electrophotographic member 10 is an induction electrodegenerally designated 14 comprising a thin insulating layer 15 coated ona transparent conductive backing electrode 16. Desirably, theelectrophotographic member 10 is supported on a suitable support member17 and the induction electrode 14 may rest on the surface of theelectrophotographic member or plate. Members 10 and 14 are illustratedas being spaced apart. It is to be understood that this spacing isextremely minute and preferably is the almost immeasurably small spacebetween two reasonably flat surfaces in normal surface contact. Likewisecertain members and structures such as, typically layer 12 are too thinto be reasonably illustrated, and therefore are shown out or proportionand diagrammatically.

Means are supplied to impose on the photoconductive insulating layer 12a pattern of light and shadow such as an optical image to be recorded.As illustrated in the figure, such means may include a lens 19 or othermember designed to project on the layer an image of an original document29 or other pattern of light and shadow to be recorded. Since theinduction electrode 14 in this embodiment is transparent it is readilyapparent that a suitable image may be projected through this transparentelectrode onto the surface of a photoconductive insulating layer 12.

A source of electric potential 21 or other suitable electric fieldgenerating means is adapted to impose an electric field of desiredpolarity between insulating layer 15 and conductive backing member 11.This potential source is indicated in the diagram as beingconventionally a battery or other source of D. C. voltage and it is tobe understood that any suitable source of electric potential may beemployed for this purpose. If desired, a conventional power supply maybe made from transformer operated rectifier systems as are well known inthe art. it is desired that there be applied between member 16 andmember 11 an electric potential such as will cause between these membersan electric field at least through the photoconductive insulating layer12 and to the insulating surface 15 whereby electric charge will beinduced to migrate through said first layer and to the surface of saidsecond layer.

In Fig. 2 there is illustrated a modification of the embodiment of theinvention shown in Fig. 1. According to this embodiment of thisinvention, an electrophotographic plate, generally designated 10,comprising a photoconductive insulating layer 12 disposed on aconductive backing member 11, is supported on a suitable support member17. Positioned parallel to and closely adjacent to the surface of theelectrophotographic plate .10 is a transparent conductive electrode 22mounted on support members 23 or the like. VPlaced on top of theelectrode 22, or on the sideaway from the electrophotographic plate, isa document 24 or other visibleor image matter which is to be reproducedaccordingito the present invention. A light source25 or other source ofactivating radiation is positioned above the assembly so as toY shinevisible light or other radiation through the copy 24 and onto thesurface of the electrophotographic member 10.

An insulating lm 27 in sheet or web form is positioned'between electrode22 and plate 10 and desirably passes from a feed roll 28 between theelectrodes and ultimatelyto a take-up coil 2,9. The web or t1mr nayoptionally pass through a suitable `xerographic processing station` 30such as for example a developing or `fixing' station or-a plurality ofsuch stations whereby desired xerographic operations may be performed onan electrostaticv latent' image. For example, xerographic stations mayinclude means for developing the-imagev or making it visible bydeposition of finely divided material in con- "o for-mity with theimage, and may includemeans for transferring the image to a'secondsurface and/or fusing .or iixing the image. 4

Electric field producing means such as an electric potential source31`is suitably connected between the electrode 22 and the backing member11 of the electrophotographic plate optionally being connected to aswitch 32 or like controlmeans to operate and control the electriclield.Y In this manner electrode 22 may be raised toa desired potentialeither positive or negative with respect to the'backingmember whereby adesired electric iield is imposed through the photoeonductive insulatinglayer 12 and Yat least to the surface of the receiving iilm27, therebyinducing electric charge to migrate to the surface of this iilrn.

Atleast during the exposure operation the induction electrode andxerographic plate are generally in normal surface contact vwith theopposite surface of iilm A27. Accordingly means, either mechanical ormanual, may be supplied to `move electrode 22 into and out of contactingposition. Conveniently this can be accomplished by manually placing theelectrode on top of the film and resting the electrode on thexerographic plate.

With respect to Figs. 1 and 2, it is apparent that there are certaingeneralities and similarities which may be noted. For example, in eachof these figures a light or radiation image is imposed on thephotoconductive insulating layer V12 and it is therefore apparent thateither the backing vmember 11v supporting `this photoconductiveinsulatinglayer or theinduction electrode 14 or 227must be transparent.;Thus, as shown in Figs. l and 2 thel induction electrode -is thetransparent member, but it is apparent that a transparent backingmemberlll would permit exposure by projection` through the Xerographicsensitive member itself. In any case, there is a'transfparent conductivemember, be it member 11 or 16 of Fig. l or both, or member AY171 or 22of Fig. 2, or both, and this member must be sufficiently conductivertopermit application of an electric eld through layer 12 and through theair gap and must permit the liow of electric charge under the iniiuenceof this field and the photoconductive action from the appropriate lightor image source. `It is Y.specifically understood and recommended vthatconductive glass members or conductively coated glass members should beso employed. trode'should have a specific resistivity of less than l()mohm cms. and preferably, less than 105 ohm cms. Desirably, this membershould be what is ordinarily considered to vbe conductive and shouldcarry a substantial flow of `current under only mild potentialdifferences.`

ffanSPafent .Suppen bases. including .glassy tr,alista-us5.`

plastic iilms and the like, and conductive transparent In general, thisconductive elecmembers coated or impregnated with conductive materialssuch as metals or ionic salts or the like Where sufficient moisture ispresent to cause the materials to be conductive. Likewise, conductiveliquids or uids may be employed, such aselectrolytic salt..solutionsor-other ionized liquids, or ionized air or other ionized gasesor Mlayerare commercially available -in the art of Xerography.l

In addition to this speciiically preferred embodiment, it is to beunderstood that therefmay be employed other photoconductive ymembersincluding anthracene, sulphur, and the like, coated on suitable backingmembers, A as well Aas photoconductive binder layers includingphotoconductive materials dispersed in a suitablebinder-.and coated on aconductive *surta-ce.V T hese materials include, for example,crystalline materials generally availableas phosphors or luminouscompounds whichl frequentlyexhibit photoconductivityfand which can beyemployed in suitable organic binders and iilm forming materialsonthesurface of photoconductive members.

In the case of photoconductive layers, according to thepresentinvention, as distinguished from the photoconductive insulatinglayers preferred in the art of Xerography in general, it is to beobserved that the layers according to the present invention are notrequired to hold anelectrostatic charge ontheir surfaceffor similarly,long periods of time, but are required on -the contrary to permitmigration of electric charge through the thickness of the layerat asubstantially diierential rate depending` on activation or nonactivationby suitable radiation. Thus, whilephotoconductive layers for xerography.in general'must `be capable of supporting an electric charge on thesurface for appreciableperiods of time to permit formation of an imageand development of the image, this is not true in the case of thepresent invention where the electrostatic image is retained fordevelopment on the insulating layer 15 of Fig. l, or the insulating film27 of Fig. 2.4 Thus xerographie members characterized by higher darkdecay or current leakage in the absence of activating radiation 'may beemployed in the present invention.

In Fig. 3f there isillustrateda .further embodiment 'of `theinventonwhereby .the jnew invention is particularly adaptedto the formation ofVX-ray or radiographic images. Asuillustrated intheflgure .aXeroradiographic plate lgenerally designated Vlitik comprising anormally insulatinglayer 12land a conductive backingimember 1 1isdisposed rover an induction electrode-14 with an insulating :transfermember 27 therebetween. A suitable power `supply Y21Y is connectedbetween conductivebacking-member 1l and the induction electrode 14 toapply a field from the one member yto the other. v

A suitable test VVobject or other material or member to be examined byX-,ray methods is placed on the conductive backing member .which therebyserves the dual pnrpose- 0f Supporting the test Obiect andsimulteneouslyoper.-

ating as the backingreleetrode forthevsystem. illustratedV l in thedrawing is a step wedge 36 such as may be particularly designedfortesting lthe Vradiograph operations. An X-r'ay source such. alsV)AC-ray tube Bul is disposed and positioned to supply penetratingradiationor X-rays indicated by dotted lines 35 which are projectedontoand through the test object to strike ltheXeroradiographic plate 10.In general the basking ,member 1 1 of the xcroraso.- graphic plate willbe a support sheet or plate of a metal 4or like elecfrialrcoaduclgrWhirl; .desirably .may rbe opaque to ordinary or visible light. It isapparent, therefore, that thepenetrating radiationsulchas X-raysstriking this bakasalembetzwill penetrate .thrqush it to eater inte thenormally insulating layer 12 disposed thereon. This normally insulatinglayer -is a l'yerof material which in ,g

the absence of activatingradiation is an insulator but which, under theactivating influence of X-rays or other penetrating radiation becomessubstantially more conductive to electricity and it may, if desired, bea photoconductive insulator such as the photoconductive insulatorsemployed according to the embodiments of the invention operating withvisible light. Alternatively, it may be such other insulating member asis substantially insensitive to visible light but activated bypenetrating radiation. Thus, for example, many insulating lms and layersbecome conductive under the action of penetrating radiation even thoughthey are not generally considered to be photoconductive and it is withinthe scope of the present invention to employ such insulating members.

In the embodiment of the invention shown in Fig. 3 it is presentlypreferred to employ a photosensitive xerographic plate comprising alayer of amorphous or vitreous .selenium disposed on and coated on aconductive backing plate comprising a sheet of brass, aluminum, or thelike.

Illustrated in Fig. 4 is a further embodiment of the invention whereinthe time or duration of exposure is controlled by controlling theapplication of potential between the induction electrode of the systemrather than by controlling the duration of exposure to light. In Fig. 4is shown a xerographic plate and an induction electrode 14 exposed to aprojected light image from an original 20 through a lens 19 or othermeans. The members and operations are in general the same as the membersand operations according to Fig. 1.

Connected between the backing member 11 of the xerographic plate and theconductive electrode 16-is a power supply or potential source 21 whichoperates through a timing switch 37 which is constructed and adapted toapply a pulse of electric potential between the two electrodes for apredetermined time as may be selected by the timing switch. According tothis embodiment ot the invention the appropriate exposure conditions areset up by projecting the desired pattern of light and shadow through thetransparent electrode 14 onto the xerographic plate 10. The timingswitch is then set and energized to apply the desired potentialdifference between the conductive members 11 and 16 for the preselectedtime as controlled by the timing switch.

The embodiment of the invention shown in Fig. 4 is particularlydesirable for use in conjunction with photo- Iconductive insulatingmembers characterized by what is lknown in the art as high dark decay.Members of this ltype are characterized by a somewhat higherconductivity in the absence of activating radiation than are membersgenerally preferred for conventional methods of xerography.Specifically, according to prior methods of xerography, it is usual toapply an electric charge to the surface of a photo-conductive insulatinglayer and to store this charge on the surface from the time the surfaceis charged until the charge is selectively dissipated by exposure tolight and inally developed by deposition thereon by chargedelectroscopic particles. The usual operations of charging, subsequentlyexposing, and then developing, require va period of time varying fromrelatively large fractions of a second up to a time as much as severalminutes and, accordingly, it is necessary that the charge be retained onthe photoconductive insulating surface in the absence of radiation orexposure for a time cycle sufficient to permit the carrying out of thesexerographic steps. Now, however, according to the embodiment of theinvention illustrated in Fig. 4 it is not necessary to place and retaina charge on the photoconductive surface for a long time as haspreviously been required in xerography. It is apparent that thisembodiment of the invention calls for activation of the photo-conductivelayer or diterential conductivity through this layer during the l timewhen the layer is both subjected to an electric eld and exposed to thepattern of radiation. .In the absence 6. of either of these conditionsthere is not the combination which leads to charge dissipation since thecharge once deposited on the insulating layer 15 is held in its imageconfiguration by the insulating characteristics of this insulating layerrather than by the insulating characteristics or absence thereof of thephotoconductive layer 12.

It is apparent, therefore, that the photoconductive layer 12, accordingto this embodiment of this invention, may be somewhat more conductive inthe absence of radiation than permissible according to prior inventionsof xerography, Thus, according to this embodiment of the invention, boththe electric image formation and other variations in electrostaticcharge pattern on insulating layer 15 are substantially restricted tothe period of time indicated by timing switch 37. An image thus formedis held on the insulating surface for a period of time sufficient topermit other xerographic steps such as development or the like,regardless of whether the photoconductive layer 12 has a suiciently lowdark decay to permit its use in other variations and modifications ofxerography.

In Fig. 5 is illustrated diagrammatically a machine for the productionof xerographic copy of suitable original material according to oneembodiment of this invention. According to this figure, a xerographiccylinder 38 is adapted to be rotated by a motor or other drive means 39optionally acting through a drive belt 40 on a pulley 41. Thexerographic cylinder in general is a drum-like or cylindrical surfacehaving at least a portion of its surface covered with a xerographicphotosensitive member as in the previous figures. At one point aroundthe circumference of the drum is a transparent induction electrode 41fitted very closely to the drum and allowing just sucient room forpassage between the electrode and the drum of a sheet or web of aninsulating lm 42 which desirably passes from feed roll 43 around guideroll 44 to a take-up roll 45. During its path of travel from feed roll43 to take-up roll 45 the lm passes between the electrode 41 and thecylinder 38 and desirably passes through one or more xcrographicstations or positions 46 which may be developing or fixing stations orthe like.

At the exposure station, this being the location of the inductionelectrode adjacent to the xerographic cylinder, is suitable apparatus ormeans for exposing the xerographic cylinder to the pattern of light andshadow to be recorded. As illustrated there is slit exposure mechanismcomprising projecting means or lens 48, an image slit 49, and aprojection slit 50, the lens being adapted to project through the imageslit a focused image of the projection slit. Material to be copied suchas for example documentary information or the like desirably in roll orweb form is adapted to be passed across the projection slit 50 forexample as the web 51 of image material passing from a feed roll 52 to atake-up roll 53 and adapted to be driven at a desired rate of speed bydrive means 54. Desirably this drive means will include suitable drivemechanism synchronized with the rate of travel of the Xerographiccylinder. In the event that web S1 is photographic microfilm or the likethe drive means may be a gear wheel 54 with teeth adapted to engage theslots of microfilm to carry the microlm past the projection slit.

The suitable D. C. voltage source is connected to the inductionelectrode in such manner as to maintain a potential difference betweenthe induction electrode and the backing member of the xerographiccylinder. This will be accomplished, of course, by means and methodssimilar to those described in the previous gure.

In use and operation the machine of Fig. 5 operates according to theprinciple of Fig. 2. Motor or drive means 39 is energized to rotate thexerographic cylinder in a clockwise direction. Image web 51 then iscarried across the projection slit at a rate synchronized with therotation of the xerographic cylinder and transaannam electrode. l Uelec-trode and the backing member of the -xerographtc,

cylinder causes formation of anelectrostatic image Von insulating web 42and this image-'bearing web `is vthen carried to the further xerographicstations where `it may be ydeveloped and fixed or otherwisetreated asdesired' to yield a xerographic print or Ato kyieldjother xerographicresults as desired including, for example, a scanning electric signal oflight. In the eventthat a xerographic print is formed the resultingprint is formed into taltefuprol-lS.

According to a further. embodiment V'ofthe invention the light image tobe recordedQis Vprojected'.throughn Y reversing means are `suppliec'ltomlakefpossible reversing Y the reverse side ofthexerographic'plateand'ifurther the potentials applied between theconductive backing electrode 1l and the induction electrode 14, Infig.f6 is illustrated this embodimentV of 'the invention. .As shown here,the induction electrodev 14 comprises a conductive surface connected toone pole of a rcversingswitchS?. Desirably Vthe conductive electrode isasupport surface such as, for example, a at metallic lsurface suitablysupported on or comprising a support Ibed plate ofthe device. Positionedover the induction electrode 14jis a xerographic plate comprising aphotoconductive insulating layer disposed -on a transparentconductive'backing electrode 11 such as, for example, va glass platewitha conductive coating 11a on the stxracet-hereOf, thiscom ductive coatingbeingv either of a transparent material or being sufficiently thin sothat it `islargely transparent to incident light. For example, theyconductive-coating may be a thin evaporated lm of aluminum or othermetal on the surface of the glass plate, or the glass may be aconductive or conductively coatedV glass as isV readily commerciallyavailable. Coated on the layer surface, or conductive surface, of theglass is aphotoconductive insulating layer 12 such as the layer 12 ofFig. l.

A sheet of an insulating material is positioned lbetween electrode 14and plate 10 or desirably a web 42 of-in sulating material is passedbetween said members passing from feed roll 43 to take-up roll 44. Theconductive backing member 11 and the inductionelectrode 14 are connected`to the output poles of lreversingV switch-f67, and the input poles areconnected to a D. C. potential-source 21. By .throwing reversing switch57 in either direction, the-potential or 'field between member 11 and;member 14 may be made positive in either direction asvdesired. In theevent that induction Velectrode 14 is .to be raised to a potential otherthan ground it generallyis desirable that this electrode be supported oninsulating supports `-58.`

In operation, the device shown in Fig.V l6 is energized by exposing thephotoconductive insulating layer `12 to the vprojected image of object20 while a potential -diierence is appliedbetweenbackingelectrode-11'and'induction electrode .14. If desired,greater intensification can be achieved .by iirst-reversing the polarityof the potential or field through reversal of switch 57 and subsequentexposure and application :of the desired potential or field.Alternatively, the yelectrostatic image placedon sheet or web 42 may betransferred yto the xerographic plate after formation of the image byreversal of the polarity such as to drive the electrostatic chargebackacross the air gap to deposit the charge on 'the'surface of layer 12. Inthis manner the electrostatic charge image can be placed either on.insulating ;lm42 or on the photoconductive layer 12 as desired, andmay.be transferred from one 'to the yother after its formation. Thus, theelectrostatic latent image may be developed or otherwiseV utilized oninsulating film-42er it "may if desired be developed or otherwiseutilized on the xerographic plate.

In Figj7 there is illustrated in greatlyy enlarged form a diagrammaticrepresentation ofv theformation-ofimages according toIone..ernbodim'critici-:this Ili-n'vention. LIt is "to beunderstood,ithatjthisgillustration is presented in anetort todeseribejthe,operationofjthe invention in vaccordance withv oneptheoryofmechanismwhich is believed toexplain image formation. It is to' berealizedghowever, u

that otherjnmehanisms of, image.. formationmay also be in accordwithexperimentally determinedfact and/that the presentV explanationispresented as one theoretical possibility.and'thepinvention is .not Vtobe limited to this expression oftheory. In Fig. 7v is laphotoconductiveinsulating layer f6.1 disposed on a conductivebackingmember 62. Spacedtherefrom byja minute-ain gap i'is an insulatinglayer 63 disposed on a conductive backing/layer .64., This conducvtivebacking llayer 64 istransparent and on the back surface thereof is an,image layer .adaptedto. interrupt the passageof light through lthelayerand to causev a light image -to be projected onto thesurface Yoftherphotoconductive insulatingl layer V61-as^indicated by arrow 66,. Itis understood that 4image 65 v`-is' -adiagrammatic representation Aofany means by projection or contact tocontrol, in

image configuration, the'exposure to activating radiation.V

Thus,as illustrated, ycertain portions of the photoconductive insulatinglayer indicated as blocks 61-[1 are struck by light and vbecomeconductive vwhereas other portions 6l-a are notstruck yby light andVremain insulating.

Asuit'able vo'ltagersourceV or voltage supply 67 is operativelyconnected between conductive backing member 64 and conductive backingmember 62 whereby a field-or potential difference-is applied betweenthese two members. In the regions of the insulating portions 61-a ofthephotoconductive insulating. layer this 'eld Ais represented byplussigns .in the induction electrode backing member 64` andbyminususigns in Vthe conductive backing member 62 of the xerographicplate, -these plus and minus signs representing .positive vpolarity andnegative polarity charge or potential. vIn :theJregions-of lightactivation, however, the seleuium-.layer'becomes conductive and,therefore, the :charge .or potential Jmigrates through the layerand canberepresented as-being Vat Lthe surface .of the Ylayer in the-areas`61'b -which are .conductivedueto the .action of light. .Itis tobe=understood, however, that 'the air gap between layer 61.and 'layer63sis Vextremely small Vand that the application. of .the relativelyhigh potentialbetween the surface of layer 61 and the conductivebackingV member .64 of the-(induction electrode will cause charge tomigrate fromthe surface of layerz61 to the surface of layer 63. Thischarge, .once deposited on the surface of layer 63, becomes rtrappedthere .because'the layer is an insulator either; in thepresence of.light or in its absence. The. result :is anelectrostaticlatent image onthe surface of lay er,63v correspondingto the .areas which areilluminated inlayer 61.

For, simplicity -ofafexplanationgin- .connection .with the currenttheory of iieldvemission, the formation of a negativefpolarity Vimagehasbeen .illustrated in .Fig 7. In

order to -explain malformation of this .image and toanalyzeit,mathematicallycertain assumptions are taken withrespecttovoltages,thicknesses, `and 4distances as well as dielectric constants.I'Forthepurposes of arnathematicalinterpretation it is therefore assumedthat apotential difference of .l 00.0 volts: is; appliedV between-electrode .64 anclelectrode 62 and that the surfaces of .layer .61 and63 arel separated:byadistanceA of.2 zmicrons. In practice a separationlof. vthis order of'distance can be achieved by placing solid particlesof thedesired diameter between the surfaces. Layer'l-is assumedto bealayer of vitreous or amorphous selenium about 20 microns thick and forthe purposes-.hereinitis .assumed that the dielectric Aconstant of thislayer,isabout 6. It is understood, KYof course, 'thatihisis areasonableapproximation of the dielectric constant ofseleniumybut thatthe `exact iigure is selected arbitrarily -because of thegfactthatthedielectric constant of seleniumvariesdependingp.on..fits;allotropic.form and perhaps also :depending,upon the conditionssuchasiield or potential gradient 'andthe like."Itris also assumed for the purposes of simplicity that layer 63 is aninsulating layer of the same thickness, namely 20 microns, and also hasthe same arbitrarily assumed dielectric constant of 6. In thissituation, then, when a 1000 volt potential difference is appliedbetween the conductive backing electrodes in the areas ofnonillumination, there will be a field of approximately 385 voltsthrough the selenium layer 61 and through insulator layer 63, and apotential difference of about 230 volts across the air gap. In the otherareas 61-b there will be substantially no field or potential differencethrough layer 61 and the field then will be such that about 375 voltswill be across the air gap and about 625 volts through insulating layer63. These voltages and fields are calculated in the assumption that theair gap is substantially a perfect insulator and this, of course, is notthe fact.

The next approximation introduced into the figures is the arbitraryassumption that a potential gradient in the order of about l volts/cm.will cause the phenomenon known as field emission in air. It has beenknown for some time that a particularly high potential gradient adjacentto a surface of a solid will cause electrons to be torn from or emittedfrom the surface. This is the phenomenon known to the art as fieldemission and measurements have indicated that this phenomenon comesabout at a potential which is generally in the order of 105 volts/cm.Accordingly, this value, as selected here arbitarily, is a reasonablerepresentation of the fact. Referring now to the previous figures, it isobserved that when a potential difference of 375 volts is applied acrossan air gap of 2 microns thickness, the potential gradient across thisair gap becomes a little less than 2X 105 volts/cm., or a little lessthan 20 times the potential gradient required for field emission.Accordingly, it is apparent that under the conditions illustrated here,field emission or other mechanism of charge transfer will occur. To putthe same facts differently, field emission or other transfer of chargemust occur when the potential gradient in the air space above thissurface exceeds about 105 volts/cm. and for a distance of 2 micronsthrough air the total potential difference will not exceed about 20volts.

lt follows that in the embodiment shown in Fig. 7 charge will migratefrom conductive backing member 62, through the conductive areas 61-b ofthe selenium photoconductive insulating layer, and across the air gap,to be deposited on the surface of layer 63 until a deposit of charge onthis layer builds up to a charge density such as to reduce the potentialgradient across the air gap to a value no higher than that required forfield emission or namely about 20 volts. This will result in theformation of a substantial charge density in the image areas on theinsulating layer 63. It is seen that for the nonconductive area 61-a ofinsulating layer 61 there cannot be the formation of an equal chargedensity in these nonimage areas of the insulating layer 63. Themigration of charge across the air gap by iield emission or othermechanism in the insulating area 61-a will build up a reverse potentialgradient across this area rapidly. When charge potential to the amountof 105 volts has migrated to areas 61-a, these areas become 105 voltsmore negative, the background areas on layer 63 become only 105 voltsmore positive and the potential across the air gap here again becomesthe 20 Volt threshold for eld emission. Using the figures presentedherein, it is seen that the image corresponding to area 61-b will be anegative polarity image on the surface of layer 63, the potential ofwhich will be equal to the applied potential of 355 volts minus thepotential of about 20 volts needed for field emission or a negativepolarity image of about 355 volts against a background potential of 105volts. Layer 63, therefore, has on it image areas charged to a potentialof about 355.volts negative polarity with background areas at apotential of about 105 volts negative polarity leaving a potentialldifterence between image and background areas of about 250 volts. Animage of this'v potential difference can be developed by the methods andmaterials of Walkup and Wise, U. S. Patent 2,638,- 416, wherein acomposition of powder particles and granular bead carrier material withthe powder particles being charged by triboelectric relationship withthe beads is rolled or cascaded across the electrostatic latentimagebearing surface to deposit the powder particles on the surface.

In carrying out the present invention care is to be taken to employ agood electrical insulator as the image surface or ilm on which theelectrostatic latent image is formed. Similarly extreme care is to betaken to place this image surface in very close proximity to thephotoconductive layer. In this connection it is to be observed that thepresent invention differs from certain prior art methods of treatingelectric charges wherein it has been attempted to induce anelectrostatic charge or charge pattern to a surface which is conductiveor which is temporarily made conductive. Thus, for example, it may bepossible to form an electrostatic image of relatively low potential onthe photoconductive insulating surface 12. According to the presentinvention, however, it is on the adjacent insulating surface and not onthe photoconductive surface that the electrostatic latent image is to beformed and the invention includes the migration of charge from onesurface to a second surface, with image formation on that secondsurface.

A brief examination of the mechanism of operation and the geometry ofthe members in terms of capacitance will illustrate the advantages thatcall for such charge transfer. In the rst place, in Fig. l, if theinduction electrode 14 is spaced at even a moderate distance from thexerographic plate 10, there is a relatively low maximum potential whichcan be formed on the photoconductive surface. For example, in Fig. 8 ofCarlson U. S. 2,297,691 there is an illustration of a system in which asubstantial air gap exists between the members and in which there isformation of an electrostatic latent image on the photoconductivesurface. However, as was done hereinbefore for the present invention, ananalysis on a mathematical basis can be made of this prior disclosure.For clarity, referring to Fig. 7 herein, it is assumed that layer 63again is a 20 micron insulating layer having a dielectric constant of 6,and in this case it is assumed that the air gap between layer 63 andlayer 61 is /lo inch air gap having a dielectric constant of l. Layer 61is again assumed to be a 20 micron layer of selenium also having adielectric constant of 6. lf in this case a potential of 1,000 volts isplaced between electrode: 64 and 62 and areas 61-b are made conductiveby light to induce charge to the surface of layer 61 and then the lightis cut off and electrode 64 removed, there will result a very smallcharge potential on these areas 61-b. Because of the air gap betweenlayers 61 and 63 of 2%@ inch or approximately times the thickness oflayer 61 itself, with the air gap having the dielectric constant of oneas against the dielectric constant of 6 for the layer 61, it is seenthat a potential of 1,000 volts through this air dielectric can induceto the surface of the conductive areas a total charge in coulombs which,after removal of electrode 64, will result in a potential difference ofonly a few volts between the surface of layer 61 and the surface ofconductive electrode 62. Therefore, the prior procedure of imageformation on the photoconductor can produce an electrostatic latentimage on the photoconductive insulating surface, but this image will beonly a few volts in magnitude. When electrode 64 is at a positivepolarity, this is a negative polarity image on layer 61.

As was seen earlier, if these two members are brought closer and closertogether there becomes finally a maximum potential in the order of about20 volts for a 2 or 3 micron spacing which can be produced on thephotoconductive insulating surface by inductionl methods. Thispotential, however, will beV comparativelyless than a potential thatmight be left onrthe background areas by charge migration away from saidareas and it is apparent that direct induction techniques are not themajor cause of image formation in that case. It is apparent thereforethat the electrostatic image which may be formed on the xerographicplate will be an image of comparatively low potential and that thepurpose of the present invention is to form images of substantiallyhigher potentials and to form such images on the adjacent insulatinglayer rather than on a photoconductive insulating layer.

Whether the mechanism of operation of the present invention is accordingto field emission or other phenomena such as air ionization or othermethods of charge migration across the air gap, extremely close spacingis necessary between the two surfaces. It has been found that for mostmaterials and surfaces the methods and apparatus of the inventionoperate at near optimum conditions when the two surfaces such as layers12 and 15 of Fig. l are placed in nominal contact. Thus for materialssuch as a selenium surface of a Xerographic plate and a polystyrene lrndisposed directly on a metallic backing member the condition of normalsurface contact hasl been found to be extremely desirable. It isunderstood, of course, that in this condition there are a relatively fewpoints of actual surface contact between the two surfaces, and at mostpoints there are air gaps in the order of about a micron. A secondsystem of somewhat preferable spacing for the surfaces described andother surfaces is a spacing system wherein a predetermined distance ofseveral microns is maintained between the two surfaces. This distance isachieved by any of several methods and the following is recommended as apreferred method. A hard, relatively clear, transparent material such asa plastic or resin is ground to a relatively uniform particle size inthe order of the size of spacingk desired between the two members. Asmall quantity of the powdered material is dusted onto one of thesurfaces in an amount to be almost invisible on the surface. The secondsurface then is placed on top of the dusted surface and the charging andexposing operations carried out as hereinbefore described. The presenceof the powder particles scattered over the surface maintains between thetwo members a spacing in the general order of the same size as theparticle diameter. In this manner distances between the two surfaces inthe order of about 2v to 5 `microns have been found nearly optimum andit is generally preferred that the two surfaces be spaced apart by adistance of less than about microns. Somewhat greater spacing may beusable and operable under certain conditions and it is generallyunderstood that the two surfaces according to the present inventionshould be spaced apart by a distance no greater than about 20 microns.It is observed that as the spacing between the surfacesincreases twocharacteristics of the image result. In` the rst place the image becomessomewhat less dense and in the second place the resolution of the imagedecreases sharply. Such decrease is significant when the two surfacesare spaced apart by as much as 10 microns. Other impaired eects oflesser signicance are apparent as spacing is increased. As a furtherobservation it is pointed out that extremely good surface contact is notto be sought after since there can, under conditions of actual contact,be substantial transfer of charge or potential in the background areasand this transfer by contact frequently will result in a mottledbackground.

In Figs. 8 and 9 are illustrated two further embodi-` ments'of inductionelectrodes which may be employed according, to the present invention.According to the embodiment of Fig. 8 a suitable induction 'electrodemay comprisefa very finemetallic or wire screen having a plastic coatingon atleast one surface thereof. Desirably,

the,worl ing surface of this electrode should be substantiallyV smoothand uniform. The electrode therefore comprises a conductive metal screenconsisting of interwoven .wires 71 imbedded in insulating plastic body72; A conductive lea-d 73 is connected to the wire screen to form ameans for applying a suitable potential to the electrode. Desirably thelead 73 is connected to a suitable wire mesh or the like and theelectrode is formedv by spraying, dipping, or painting the insulatingfilm material onto the screen or mesh. It is desirable that the wires berelatively fine and the screen be substantially transparent totransmitted light, and be extremely ne and uniform. A ne screen such asa 200 mesh` or finer sieving-screen is excellent for the purpose, so,that the screen or mesh pattern does not become significant as part of;the electrostatic latent image, and so that the screen or mesh patterndoes not unduly affect the transmission of a projected light image tothe electrode.

Iny Fig. 9 is illustrated a cylindrical induction electrode designated75. The electrode comprises a glass or other transparent supportcylinder 76 having a coating 77 on its surface comprising a thin layerof metal such asaluminum deposited in a uniform film, for example,tbyevaporation on to the glass surface. Positioned on the metal film is alayer of an insulating material 78. Suitable support means are providedsuch as, for example, spokes 79 connecting the cylinder to a hub 80which is adapted to be rotated around a shaft or axle or the like. As isapparent the electrode of the type described herein is particularlyadapted to be used in conjunction with a continuous machine such as isdisclosed in Fig. 5. If desired suitable image sources such as a lightsource or a reflecting mirror, prism, lens, or the like, may be mountedwithin the cylindrical induction electrode so that the inductionelectrode operates as a combination of the electrode 41 and lenserorimage member 48 of Fig. 5.

It is to be understood that variations and modifications may be madewithout departing from the scope of the invention, and that thedisclosure herein is to be taken as being illustrative of the inventionand not in limitation thereof.

This is a continuation-in-part of copending application Serial No.368,408, filed July 16, 1953.

What is claimed is:

1; The method of forming an electrostatic image comprising varyingelectric charges on an insulating surface said method comprisingpositioning an insulating surface over a conductive electrode and invirtualtcontact with the surface of a normally insulating layer disposedon a conductive backing support, said insulating surface beingpositioned in face-to-face relationship with the surface of the normallyinsulating layer and separated therefrom by a minute gas gap, applyingan electric eld above the threshold of eld discharge for the particulargap distance involved between the surfaces in virtual contact throughsaid normally insulating layer and to the insulating surface to bringabout electric breakdown of the gas gap between the surface of thenormally insulating layer and the insulating surface to cause ionicmovement in the gap for charge deposition, as controlled by the electricfields, on the insulating surface in conformity with a pattern ofpenetrating radiation being recorded while exposing the normallyinsulating layer to said pattern of penetrating radiation, said appliedelectric eld being of a sutcient intensity in the .absence ofphoto-emission for charge deposition on the insulating surface.

2. The method of forming an electrostatic image comprising varyingelectric charges on an insulating surface over a conductive electrodeand spaced from the surface of an insulating layer carrying anelectrostatic image, the insulatinglayer being disposedron a conductivebacking support, the insulating surface and the image bearinginsulatinglayer being spaced apart by a minute gas gap while inV face-to-facerelationship, Vsaid method comprising applying a high enough electricfield through theV asaasie insulating image bearing layer and to theinsulating surface for the particular spacing involved between theinsulating surfaces to bring about electric breakdown of the gas gapbetween the surface of the insulating image bearing layer and theinsulating surface to cause ionic movement in the gap for chargedeposition as controlled by the electric fields and electrostatic imageformation on the insulating surface in conformity with the electrostaticimage of the insulating image bearing layer, said applied electric fieldbeing of a suicient intensity in the absence of photo-emission forelectrostatic image formation on the insulating surface.

3. The method of forming an electric image, comprising varying electriccharges, on a first insulating surface in virtual contact with anelectrostatic image bearing second insulating surface and inface-to-face relationship thereto, said method comprising applying anelectric field above the threshold of field discharge between the twoinsulating surfaces to cause electric breakdown of the gap between thesurfaces in virtual contact to create ionic movement for chargedeposition as controlled by the electric fields and electrostatic imageformation on the first insulating surface in conformity with theelectrostatic image on the second insulating surface, said appliedelectric eld being of sufficient intensity in the absence ofphotoemission for electrostatic image formation on the first insulatingsurface.

4. The method of forming an electrostatic latent image comprisingvarying electric charges on an insulating surface said method comprisingpositioning an insulating surface in virtual contact with the surface ofa photoconductive insulating layer disposed on a conductive backingsupport, said surface being disposed and positioned in face-to-facerelationship with the surface of the photoconductive insulating layer,applying a high enough electric field through said photoconductiveinsulating layer and to the insulating surface to bring about electricbreakdown of the gap between the surface of the photoconductiveinsulating layer and the insulating surface to cause ionic movement inthe gap for charge deposition, as controlled by the electric elds on theinsulating surface in conformity with a pattern of activating radiationbeing recorded while exposing the photoconductive insulating layer tosaid pattern of activating radiation, said applied electric field beingof a suicient intensity in the absence of photoemission for chargedeposition on the insulating surface.

5. The method of forming an electrostatic latent image comprisingvarying electric charges on an insulating surface comprising positioningsaid surface over a conductive electrode and in virtual contact with thesurface of a photoconductive insulating layer disposed on a conductivebacking support, said surface being positioned in face-to-facerelationship with the surface of the photoconductive insulating layerand spaced apart therefrom by a minute gas gap, applying an electricfield above the threshold of field discharge through saidphotoconductive layer and to the insulating surface while exposing thephotoconductive layer to a pattern of radiation to be recorded to causeelectric breakdown of the gas gap between the surfaces in Virtualcontact to create ionic movement 1n the gap for charge deposition on theinsulating surface, as controlled by the electric fields, in conformitywith the pattern being recorded, said applied electric field being ofsuicient intensity in the absence of photoemission for charge depositionon the insulating surface.

6. The method of claim 5 in which the insulating surface and the surfaceof the photoconductive layer are spaced apart by a gap distance of up toabout microns.

7. The method of claim 5 in which at least one of the two paths to thephotoconductive insulating layer, namely through the conductiveelectrode and the insulating surface or through the conductive backingsupport, is transparent and in which the pfattern of light and shadow tobe recorded is directed thrmugh the transparent path.

8. The method of claim 5 in which the photoconductive insulating layersubjected to exposure comprises photoconductive insulating selenium.

9. The method of forming anelectrostatic latent image comprising varyingelectric charges on an insulating surface, said method comprisingpositioning the insulating surface in virtual contact with the surfaceof a photoconductive insulating layer and in facc-to-face relationshipthereto while spaced apart therefrom by a minute gap, applying anelectric field above the threshold of field discharge through saidphotoconductive insulating layer and to the insulating surface Whileexposing the photoconductive layer to a pattern of activating radiationto be recorded, said electric field being suicient in the absence ofphotoemission to cause electric breakdown of the gap between thesurfaces in virtual contact to create ionic movement in the gap forcharge deposition on the insulating surface, as controlled by theelectric fields, in conformity with the pattern being recorded.

10. The method of forming an electrostatic image cornprising varyingelectric charges on an insulating surface overlying a conductiveelectrode and spaced from the surface of an insulating layer carrying anelectrostatic image, the insulating layer being disposed on a conductivebacking support, the insulating surface and the image bearing insulatinglayer being spaced apart by a minute gas gap while in face-to-facerelationship, said method comprising applying an intense electric fieldthrough the insulating image bearing layer and to the insulatingsurface, and while the field continues to be applied separating theinsulating surface from the insulating layer, said applied electricfield being above the threshold of field discharge and of a sufficientintensity to cause electric breakdown of the gap between the insulatingsurface and the image bearing insulating layer to create ionic movementand charge deposition as controlled by the electric fields to form, inthe absence of photoemission, an electrostatic image on the insulatingsurface in true conformity with the electrostatic image on theinsulating layer.

l1. The method of forming an electrostatic latent image comprisingvarying electric charges yon an insulating surface said methodcomprising positioning an insulating surface in virtual Contact with thesurface of a photoconductive insulating layer disposed on a conductivebacking support, said surface being disposed and positioned in face toface relationship with the surface of the photoconductive insulatinglayer, applying an intense electric eld through said photoconductiveinsulating layer and to the insulating surface while simultaneouslyexposing the photoconductive insulating layer to a pattern of activatingradiation, and while the field continues to be applied separating theinsulating surface from the photoconductive insulating layer, saidapplied electric field being above the threshold of eld discharge and ofa sufficient intensity to cause electric breakdown of the gap betweenthe insulating surface and the image bearing insulating layer to createionic movement and charge deposition as controlled by the electricfields to form, in the absence of photoemission, an electrostatic imageon the insulating surface in true conformity with the radiation patternto which the photoconductive insulating layer was exposed.

References Cited in the le of this patent UNITED STATES PATENTS2,221,776 Carlson Nov. 19, 1940 2,277,013 Carlson Mar. 17, 19422,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 19442,666,144 Schaffert et al. Jan. 12, 1954 2,693,416 Butterfield Nov. 2,1954 2,701,764 Carlson Feb. 8, 1955 FOREIGN PATENTS 188,030 GreatBritain Oct. 23, 1922

